6 ideas
An offer listed web-only for $59, print-only for $125, and print-plus-web for $125. Because the print-only option was plainly worse than the bundle at the same price, most students chose the bundle; when the decoy was removed, most switched to the cheaper web-only option. An inferior option that nobody picks can still steer choices by making one alternative look like an easy win.
People rarely judge value in absolute terms. They compare options to nearby alternatives that are easy to compare and ignore options that are hard to compare. So the set of options on offer, not just the options themselves, determines what feels like a good deal.
An arbitrary first number, such as the last two digits of a social security number, sets how much people are willing to pay. Their later valuations then stay internally consistent with that starting point. Prices feel logical relative to each other, but the whole structure rests on an anchor that was random.
Cutting a price from one cent to zero shifts choices far more than any other one-cent change. When chocolates cost 15¢ and 1¢, most people chose the premium one; when both dropped by one cent to 14¢ and free, most switched to the free one. A free item seems to remove any possible downside, so people overvalue it relative to its actual net benefit.
Interactions run under one of two norm systems: social norms based on reciprocity and goodwill, or market norms based on explicit payment. Adding money or fines can flip a relationship into market mode, as when a daycare fine for late pickup led parents to arrive late more often. Once social norms are displaced, they are hard to restore even after the payment is removed.
The book reports that asking people to recall moral codes, such as the Ten Commandments, before a self-scored test eliminated the modest over-reporting that normally occurs. Its explanation is that people cheat only as much as they can while still seeing themselves as honest.